Many newer vehicles that are likely to be used to tow trailers (i.e., tow vehicles) come with integrated brake controllers designed to control the electric brakes on a trailer. Typically, the electric brake signal emanating from these integrated brake controllers is not a DC voltage, but is instead a train of pulses with widths that change with the amount of brake force desired. Stated another way, brake signaling is via pulse-width modulation (PWM). An advantage of PWM is that the electronic brake controller circuitry is simplified and inherently more reliable.
As brake controllers have become more sophisticated, they have added load monitoring circuits to identify a properly connected trailer. Newer brake controllers are not only looking for a load but also the “signature” of a trailer's electric brakes.
Electric trailer brakes use an electromagnet to supply the braking energy to the trailer. Electrically, these brake magnets have the same electrical signature as a typical inductor. When a DC voltage is placed across an inductor, the current rises from zero amps to its final value over a period of time. Most of the newer electric brake controllers are looking for this rising current function as an indication that the electric brakes are properly connected and functioning. The controllers periodically send a short power pulse to the brakes looking for the brake magnet signature. If the signature is lost, the tow vehicle will warn the operator that the trailer braking is either compromised or non-existent. In some cases, the brake controller will stop functioning all together.
Both the pulsed brake signal and the load detection circuitry in modern brake controllers make these devices more effective at accomplishing their tasks, but at the same time, more difficult for a technician to test for proper operation.